Nonvolatile memory elements are used in systems in which persistent storage is required. For example, digital cameras use nonvolatile memory cards to store images and digital music players use nonvolatile memory to store audio data. Nonvolatile memory is also used to persistently store data in computer environments.
Nonvolatile memory is often formed using electrically-erasable programmable read only memory (EEPROM) technology. This type of nonvolatile memory contains floating gate transistors that can be selectively programmed or erased by application of suitable voltages to their terminals.
As fabrication techniques improve, it is becoming possible to fabricate nonvolatile memory elements with increasingly smaller dimensions. However, as device dimensions shrink, scaling issues are posing challenges for traditional nonvolatile memory technology. This has led to the investigation of alternative nonvolatile memory technologies, including resistive switching nonvolatile memory.
Resistive switching nonvolatile memory is formed using memory elements that have two or more stable states with different resistances. Bistable memory has two stable states. A bistable memory element can be placed in a high resistance state or a low resistance state by application of suitable voltages or currents. Voltage pulses are typically used to switch the memory element from one resistance state to the other. Nondestructive read operations can be performed to ascertain the value of a data bit that is stored in a memory cell.
Resistive switching memory elements typically include multiple metal oxide and nitride films between two electrodes as a resistive switching layer. The films are typically deposited as a stack of films. These multiple metal oxide and nitride films exhibit bistability, and can be placed in the high resistance state or low resistance state by applying the suitable voltages or currents. Because multiple films are used to form the resistive switching portion of the memory element, the production complexity and cost of current memory elements is high.
In addition, current resistive switching memory elements are formed in multiple semiconductor processing chambers. Each time the deposited film is moved from one chamber to another, contamination such as hydrocarbons can be introduced onto the surface of the device. These hydrocarbons can cause a bad interface between the electrodes and the resistive switching layer, which degrades the device performance and lowers the yield. Moreover, if the materials used for the electrodes and the switching layer are very different in the film composition, they can be incompatible. In particular, if the materials used for the electrode and resistive switching layer are not from the same family, such as, for example, using TiN for the electrode and HfOx for the switching layer, there are typically incompatibility issues due to interfacial mixing. These can all affect the required currents and voltages necessary to reliably set, reset and/or determine the desired “on” and “off” states of the device, increase the overall power consumption of the memory chip, increase resistive heating of the device and increase cross-talk between adjacent devices. Further developments and improvements are needed.